Field of Technology
This disclosure relates to an organic electroluminescent display device, and more particularly to an organic electroluminescent display device capable of improving reliability of a contact portion between low-potential supply line and a cathode electrode of an organic light emitting diode.
Discussion of the Related Art
Recently, various flat panel display devices capable of reducing weight and volume, which are disadvantages of CRT (Cathode Ray Tube), have been developed. The flat panel display devices may be a liquid crystal display device (LCD), a field emission display device (FED), a plasma display panel (PDP), an organic electroluminescent display device and so on.
The organic electroluminescent display is a self-emissive display device which excites an organic compound to emit light. It does not require a backlight used in the LCD, so its thickness and weight can be reduced and can simplify the process. Also, the organic electroluminescent display device is widely used because it can be manufactured at low temperature, has a high response speed of 1 ms or less, and has properties such as a low power consumption, a wide viewing angle, and a high contrast.
The organic electroluminescent display device includes an organic light emitting diode (OLED) that converts electric energy into light energy. The organic light emitting diode includes an anode electrode, a cathode electrode, and an organic light emitting layer disposed between the anode electrode and the cathode electrode. Holes are injected from the anode electrode and electrons are injected from the cathode electrode. An exciton is generated when the holes are injected into the organic light emitting layer through the anode electrode and the electrons are injected into the organic light emitting layer through the cathode electrode. The excitons emit light while emitting energy.
The organic electroluminescent display device comprises gate lines, data lines, common power lines, and pixels defined by their crossings. Each pixel includes a switching thin film transistor, a driving thin film transistor, a storage capacitor, and an organic light emitting diode. The switching thin film transistor is turned on when a scan pulse is supplied to the gate line to supply the data signal supplied to the data line to the storage capacitor and a gate electrode of the driving thin film transistor. The driving thin film transistor controls the current supplied from a power supply line to the organic light emitting diode in response to the data signal supplied to the gate electrode, thereby controlling the amount of light emission from the organic light emitting diode. The storage capacitor charges the data supplied from the data line through the switching thin film transistor so that the driving thin film transistor can maintain the emission of the organic light emitting diode by supplying a constant current until the data signal of the next frame is supplied even if the switching thin film transistor is turned off.
Hereinafter, a related art organic electroluminescent display device will be described with reference to FIG. 1.
FIG. 1 is a cross-sectional view showing a part of a related art organic electroluminescent display device.
Referring to FIG. 1, a related art organic electroluminescent display device includes a display panel 10, a control PCB 20, source PCBs 22, gate drivers GD, and chip-on films 24.
The chip-on films 24 are electrically connected to pads of the source PCBs 22 and data pads of the display panel 10. On the chip-on film 24, a source integrated circuit (hereinafter, referred to as a source IC) SIC as a source driver circuit is mounted.
The source PCB 22 is provided with various wirings for supplying digital video data, timing control signals and power supply voltages required for the display panel from the control PCB 20.
A control circuit and a data transfer circuit are mounted on the control PCB 20. The control PCB 20 supplies the timing control signals for controlling the operation of the source ICs SIC together with the digital video data and the power supply voltages to the source ICs SIC of the chip-on films 24 through the source PCBs 22.
In FIG. 1, signal lines for supplying timing control signals, data signals and so on, a supply line for supplying a high-potential voltage, and the like are omitted in order to avoid complication of the drawing. Only a low potential voltage supply line VSSL for supplying a low potential voltage VSS to the cathode electrode CAT of the organic electroluminescent display device is shown.
In the related art organic electroluminescent display device, the low potential voltage VSS is supplied to the cathode electrode CAT formed on the display panel through the control PCB 20, the source PCBs 22 and the source ICs SIC as shown in FIG. 1.
Low potential voltage supply lines VSSL for supplying the low potential voltage VSS are connected to the cathode electrode CAT via the control PCB 20, the source PCBs 22 and the chip-on film 24.
The low potential voltage supply lines VSSL are formed when the gate lines are formed in the display panel 10 the organic electroluminescent display device in order to reduce the number of process steps, and the cathode electrode CAT is formed when the organic light emitting diodes are formed after forming the data lines. As a result, the low potential voltage supply lines VSSL and the cathode electrode CAT are formed in different layers. Accordingly, in order to connect the low potential voltage supply lines VSSL and the cathode electrode CAT to each other, a plurality of contact holes CH must be formed in layers existing between them.
In this construction, there are an insulation layer covering the low potential voltage supply lines VSSL, a first auxiliary cathode electrode formed on the insulation layer on which the source/drain electrodes of the thin film transistor arranged in the display area (active area) of the display panel are formed, a passivation layer covering the first auxiliary cathode electrode, an overcoat layer for planarization disposed on the passivation layer, and a second auxiliary cathode electrode formed on the passivation layer on which the pixel electrodes are arranged between the low potential voltage supply lines VSSL and the cathode electrode CAT. In the contact areas between the low potential power supply voltage lines VSSL and the cathode electrode CAT, the first auxiliary cathode electrode is connected to the low potential voltage supply lines VSSL via through-holes passing through the insulating layer. Also, the passivation layer and the overcoat layer on the low voltage supply lines VSSL are removed so that the first and second auxiliary cathode electrodes are directly connected, and the cathode electrode is disposed on the second auxiliary cathode electrode.
However, when the cathode electrode is disposed on the second auxiliary cathode electrode, there is a lifting phenomenon between the second auxiliary cathode electrode and the second auxiliary cathode electrode because the second auxiliary cathode electrode formed of a transparent conductive material such as indium tin oxide (ITO) and the overcoat layer formed of the organic insulating material disposed thereunder are not in good contact with each other. For this reason, when the second auxiliary cathode electrode is etched, the end portion of the second auxiliary cathode electrode may have a reverse tapered shape.
Therefore, when the cathode electrode is formed on the second auxiliary cathode electrode, a hole is formed in the cathode electrode or a thickness of the cathode electrode is thin due to the reverse tapered shape of the second auxiliary cathode electrode, and the resistance of the portion corresponding to the reverse tapered shape becomes higher. Therefore, when the display panel is driven for a long time, there is a problem that the display panel is damaged.